1. Field of the invention
The present invention relates to an optical disc and an optical disc player. More specifically, the present invention relates to an optical disc such as compact disc (CD), laser disc (LD), photo-magnetic disc and etc., and a reproducing apparatus thereof.
2. Description of the Related Art
In a prior art 12 cm CD with a conventional standard format, as shown in FIG. 1, a shortest pit length (a pit length of 3 T) of an EFM demodulation is approximately 0.97 .mu.m (at a time that a reproducing time is 60 minutes), a pit width is approximately 0.7 .mu.m, and a track pitch is approximately 1.6 .mu.m. In a player for reproducing a conventional standard format CD, the CD is rotated at a linear velocity of 1.2-1.4 m/s, and by utilizing an objective lens having a numerical aperture of 0.45, a laser having a wavelength of 780 nm and a spot size of 1.4 .mu.m is irradiated onto a track of the CD, whereby audio data is reproduced at a transfer rate of 1.4 Mbps.
In recent years, in accordance with the development of a short wavelength laser, it becomes unnecessary to make a track pitch equal to the track pitch of the prior art. On the other hand, in order to record a video signal, it is necessary to increase a recording density on the disc. Especially, in a case of a digital video disc and etc. utilizing MPEG2 that will become a standard in the near future, it is desired that a high-density reproduction of four times the density of the conventional standard format becomes possible.
As a response to such a requirement, in an article titled as "Development of Quadruple Density Compact Disc", p.p. 174-177 of a collection of papers in JAS conference '90, a CD in which a recording density is made quadruple the recording density of the conventional standard format is proposed, in that by utilizing an argon ion laser having a wavelength of 457.9 nm, and an objective lens having a numerical aperture of 0.93, a track pitch is made 0.8 .mu.m and a recording linear velocity is made 0.7 m/s. The quadruple density CD is reproduced with a beam spot size of 0.75 .mu.m by utilizing an air-cooled compact argon laser having a wavelength of 488 nm and an objective lens having a numerical aperture of 0.6.
According to the prior art, it is possible to make a large recording density of a CD. However, in the prior art, since the air-cooled compact argon laser is utilized for reproducing the CD, it is impossible to make a disc player compact. In other words, the prior art can not be utilized for a miniaturized disc player.
Furthermore, in the prior art, since the argon laser having a wavelength of 488 nm is used, it is impossible to reproduce a CD on which data is recorded according to a conventional standard format. More specifically, in general, a pit is recorded with a pit depth of .lambda./4 n (.lambda.=wavelength, and n=refractive index .apprxeq.1.5). Therefore, in a case of the conventional standard format, a pit is recorded with a pit depth (approximately 112 nm) applicable to a wavelength of 670 nm. On the other hand, in a case of a wavelength of 488 nm, a pit depth of approximately 81 nm is suitable. Therefore, if the CD of the conventional standard format is reproduced by the argon laser having a wavelength of 488 nm, a phase difference between an irradiated light and a reflected light at a surface of the disc does not become 180 degrees, and therefore, a level of a reproduced signal becomes small. As a result, the CD of the conventional standard format can not be reproduced by the argon laser having a wavelength of 488 nm. In other words, in the prior art, there is no compatibility for a high-density disc and a low-density (conventional standard format) disc.